Under Option B+, the new approach for prevention of mother to child transmission (PMTCT) recommended by the World Health Organization and adopted by Tanzania, all HIV-infected pregnant women begin lifetime antiretroviral therapy (ART) during pregnancy, regardless of clinical staging. This protocol holds great promise for improving women's long-term health and preventing forward transmission, but there are concerns about implementation challenges, particularly regarding women's engagement in care in the postpartum period. Longitudinal research in several African settings suggest that women are at high risk of HIV care drop- out in the postpartum period. The goal of the proposed study is to examine the implementation of Option B+ in three sites in the Kilimanjaro Region of Tanzania, to understand factors at both the facility and patient levels that support or obstruct women's engagement with HIV care following childbirth, and to identify opportunities to support the successful implementation of Option B+. This goal will be accomplished through three aims and related methods, based on the Consolidated Framework for Implementation Research. First, we will examine facility-level factors influencing postpartum care engagement. Methods will include clinical observations to understand process of implementation, 30 key informant interviews to explore provider perspectives and experiences, and review of 512 patient records to document care engagement outcomes. Second, we will examine patient-level factors influencing post-partum care engagement. Methods will include a cohort of 100 HIV-infected women, enrolled during pregnancy and followed at 3 and 6 months postpartum to identify predictors of postpartum HIV care engagement, and qualitative in-depth interviews with a subset of 24 women to provide further context to the data. Third, we will identify opportunities to improve implementation of Option B+ so that it enhances and promotes postpartum HIV care. Methods will include 9 focus group discussions with providers and patients, and a stakeholder forum to share and discuss the study findings. The proposed study will address the critical need to support engagement with lifelong ART for women beyond the pregnancy period, and therefore support the success of the Option B+ protocol for improving the coverage of ART in high- infection, low-resource settings. Through multi-methods data collection and triangulation across stakeholders, the study has the potential to uncover innovative implementation solutions, which can be further refined and tested in a multi-site intervention trial. Kilimanjaro Christian Medical Center (KCMC) in Moshi, Tanzania is well positioned to lead an implementation science study on this topic. The NIH and PEPFAR have made strategic investments at KCMC to develop its capacity as a research center of excellent, including funding from five NIH institutes, and MEPI and AITRP capacity-building awards. This study will build upon those investments to further strengthen KCMC's capacity to conduct meaningful implementation science research in Tanzania.